1. Field of the Invention
The present invention relates to a variable valve train for an internal combustion engine, which continuously controls valve drive outputs.
2. Description of the Related Art
A reciprocal engine (internal combustion engine) installed in an automobile is provided in its cylinder head with a variable valve train that at least continuously controls the valve properties of an intake valve for the purpose of addressing engine exhaust and improving pumping loss.
As a variable valve train of this type, a variable valve system is applied, in which at least a valve lift amount of the intake valve is continuously changed to allow an intake air amount. Many of the variable valve systems have a structure in which the valve drive outputs (valve lift amount, opening/closing timing, valve open duration, etc.) are continuously varied according to a swivel displacement that is inputted from a control shaft (see Unexamined Japanese Patent Publication No. 2005-299536, for example).
Inputs of the control shaft of the variable valve train are generally achieved through a structure in which the cylinder head is attached with an electric motor serving as a rotation drive source and a transmission mechanism for transmitting to the control shaft the control rotation that is outputted from an output shaft of the motor. Structures of variable valve trains include, for example, a structure in which a unit obtained by combining a ball screw shaft and an electric motor for driving the screw shaft is fixed to a cylinder head, and the control rotation of the motor is transmitted to a control shaft through a ball nut that is screwed onto the ball screw shaft (see Unexamined Japanese Patent Publication No. 2004-332549), a structure in which a unit obtained by combining a screw shaft and an electric motor for driving the screw shaft is fixed to a cylinder head, and the control rotation of the motor is transmitted to a control shaft through a link that is screwed onto the screw shaft (see Unexamined Japanese Patent Publication No. 2005-42642), etc.
A variable valve train is required to be easily repairable and replaceable. Particularly, an electric motor, being an important component of the variable valve train, preferably can be quickly repaired or replaced.
However, the electric motor of the variable valve train is installed in a transmission mechanism so as to be unmistakably positioned together with the ball screw shaft or the screw shaft(see Unexamined Japanese Patent Publications No. 2004-332549 and No. 2005-42642). For this reason, once the motor is removed from the transmission mechanism for repair or replacement, it is difficult to set up the motor again to be aligned with the axis of the ball screw shaft or of the screw shaft with high precision. Particularly if input shafts of the transmission mechanism, including the ball screw shaft and the screw shaft, are incorrectly positioned when the motor is placed back to the cylinder head after repair or for replacement, excessive friction is likely to be caused in sliding portions of the transmission mechanism. It is required for a variable valve train that continuously varies the opening/closing timing and the valve lift amount of an intake (or exhaust) valve to have high response in order to quickly and continuously implement variable control on the opening/closing timing and the valve lift amount according to an engine load state (operation state of an automobile). However, if the excessive friction is generated, it deteriorates the control response, and engine performance cannot be fully exerted. The excessive friction also influences the durability of the variable valve train.
One idea for solving this problem is to detachably fix the motor to a cylinder block as a separate body from the transmission mechanism, instead of forming a unit construction.
However, the bothersome axis alignment for aligning the axis of the output shaft of the motor with an input shaft of the transmission mechanism cannot be eliminated simply by making the motor detachable. It is then impossible to avoid a deterioration in response of control and an influence on the durability of the variable valve train.
Furthermore, the motors of the variable valve trains are located under the utilized transmission mechanisms (see Unexamined Japanese Patent Publications No. 2004-332549 and No. 2005-42642). Therefore, the detachment of the motors is likely to incur lubricating oil leakage, which generates environmental load.